Fourier transform infrared (FTIR) spectrometry was developed to overcome limitations of dispersive spectrometry techniques, particularly the slow scanning process. With FTIR, all infrared (IR) frequencies can be measured simultaneously, rather than individually, with a simple optical device referred to as an interferometer. An interferometer produces a unique signal containing all IR frequencies “encoded” within it. This signal can be measured very quickly, e.g., within approximately one second, thereby reducing the time element per sample to a matter of only a few seconds rather than several minutes.
Most interferometers employ a beamsplitter which receives an incoming IR beam and divides it into two optical beams. One beam is reflected by a flat mirror which is fixed in place. The other beam is reflected by a movable flat mirror which is controlled to move back and forth over a short distance (e.g., a few millimeters). The resulting two reflected beams are recombined when they meet back at the beamsplitter.
With one beam traveling a fixed path length and the other traveling a constantly changing path length (due to the mirror movement), the recombined signal exiting the interferometer is the result of these two beams “interfering” with each other. This resulting signal is referred to as an interferogram and has a unique property in that every data point (as a function of the moving mirror position) forming the signal has information about every IR frequency received from the IR source. As a result, as the interferogram is measured, all frequencies are measured simultaneously, thereby enabling extremely fast measurements.
To perform an analysis, a user requires a frequency spectrum (as a plot of received IR signal intensity at each frequency) to make an identification, which means that the measured interferogram signal cannot be interpreted directly. A form of “decoding” the individual frequencies is needed, and is done by performing a Fourier transformation with a computer which then presents the user with the desired spectral information for analysis.
In prior art FTIRs, the IR light spectra collected by the sample detector is shaped by the FTIR instrument sampling optics, and by the sample light absorption if it is placed in the sampling optics of the instrument. Since user is only interested in the light absorption caused by the sample, what is needed is a relative scale for the absorption intensity, which requires a background spectrum to be measured, normally with no sample in the beam. (This background spectrum represents characteristics of the instrument itself and the IR signal path within the instrument, e.g., water and/or carbon dioxide (CO2) in the air through which the IR signal travels.) This measured background is then compared to measurements made with the sample in the beam to determine the relative transmittance (e.g., in terms of a percentage by ratioing out the shape of the stored background scan collected before the sample was placed in the sampling optics). This technique results in a measured spectrum with instrumental characteristics removed. Thus, all spectral features which are present are due to only the sample. Typically, a single background measurement is made and stored for use in multiple subsequent sample measurements.
However, performing such background spectrum measurements take extra time and is not possible if the sample cannot be removed. Hence, in cases of a fixed sample, a stored background spectrum of some age must be used. A reference detector can be used to collect some of the IR signal before it reaches the sample to enable verification that the system is scanning properly. However, since most of the light is sent to the sample, the normal reference signal is weak and can only be used for low grade verification that the system is collecting data but cannot verify that the sample data is of good quality.
Additionally, problems arise when something in the light path changes between the time the background spectrum was collected and the sample data was collected. The changes show up as errors in sample data. Exemplary changes include portions of the sample left over in the optical path due to poor cleaning. Additionally, dirt, dust, and optical mechanical damage in the sampling optics can affect the light getting to the detector on different sample runs over time, and after the testing of many samples. Temperature changes can also affect the optics by changing the light at the detector. Rapid changes in water vapor and CO2 in the purge air in the light beam path can create problems as well. Purge gas can be used to reduce water vapor error in the data; however, the use of such purge gas can be expensive.
Prior art systems typically run a background spectrum measurement just before collecting sample data, and accept some water vapor error in data as an accepted standard process. It would be desirable to provide a device that reduces the need to use purge gas and the need for a separate background collection for each sample.